The present invention relates to controllers for radio frequency (RF) power generators, and more particularly to controllers for RF power generators with reduced cable length sensitivity.
Many radio frequency power generators include controllers that regulate RF output power and prevent amplifier damage due to load mismatch, excessive supply voltage, and excessive operating temperature. The controllers also minimize damage after failure of one or more of the power devices. FIG. 1 shows a typical radio frequency (RF) power generator 10 that includes a power module 11 and a controller 12. The power module 11 receives signals from RF exciter 14, amplifies the signals, and delivers the signals to a load 16. The power module 11 includes a driver 18 and a final amplifier 20. The power module 11 receives DC power through a cable 24 that is coupled to a remote battery 26 with a ground return. The cable 24 may have substantial distributed impedance. The controller 12 includes an amplifier 30, a frequency compensation capacitor 34, and a buffer 38. The controller 12 receives control inputs 40 and feedback signals 42 and produces a control voltage 44 that varies the gain of the driver 18.
The controller 12 regulates output power during normal conditions and protects the power module 11 during abnormal conditions. The controller 12 employs negative feedback to diminish an error between the greatest feedback signal and a reference input that has been selected according to nominal operating levels of the feedback transducers. Feedback signals from the power module 11 include forward and reverse power signals 50 and 52 that are generated by RF detectors 54 and 56. The detectors 54 and 56 are typically coupled to sampling arms of a directional coupler 60. Other feedback signals include a temperature signal 62 from a thermistor 64 that is thermally coupled to the final amplifier 20. Differential voltage feedback signals 66 and 70 are proportional to DC input current to the power module 11 (through a current-sampling resistor 72). A drive signal 74 feeds back the drive current to the final amplifier 20. A feedback signal 76 feeds back the control voltage 44 that is supplied to the driver 18.
Under normal conditions, all of the feedback signals except the forward power signal 50 are small. The controller 12 increases the control signal 44 until the forward power signal 50 becomes approximately equal to a reference setpoint. Under abnormal conditions, the other feedback signals increase and exceed the forward power signal 50. For example, the reverse power signal 52 increases when the load 16 becomes mismatched or is removed. Increasing the drive signal 44 without a corresponding increase in the forward power signal 50 indicates load mismatch or malfunction of the final amplifier 20. Excessive control voltage for a given output power typically corresponds to a problem in the driver 18. Low DC input current indicates load mismatch, a faulty driver 18, or a faulty final amplifier 20. High DC input current or a high final amplifier 20 temperature indicates that the controller 12 should reduce forward power demands on the power module 11. When one or more of these conditions occur, the controller 12 reduces the drive signal 44 to the power module 11 to keep the largest feedback signal approximately equal to the reference setpoint.
Conventional methods for protecting the RF power generator 10 also typically employ a set of measured generator parameters and hard setpoint limits for each parameter. For example, maximum reflected power is limited to 600 watts (W), maximum power amplifier (PA) current is limited to 40 Amps (A), and maximum PA dissipation to 1800 W. This protection technique is effective in protecting the RF power generator 10 from adverse loads but does not give repeatable performance when a length of a cable between the RF power generator 10 and the load 16 is varied.
Referring now to FIG. 2, a simplified power generator control system 100 according to the prior art is illustrated. The RF power generator control system 100 includes a power module 102, a RF sensor 104, a load 106, and a controller 108. The power module 102 generates power module feedback signals 109 (such as PA supply current 110 and device temperature 114). The RF sensor 104 generates RF sensor feedback signals 115 (such as forward and reverse power 116 and 118). The power module feedback signals 109, the RF sensor feedback signals 115, and an external setpoint signal 120 are input to the controller 108. The controller 108 generates a power module setpoint signal 124 that is input to the power module 102. The power module setpoint signal 124 controls the forward power output by the power module 102.
The basic control technique is to provide negative feedback signals from various detectors (such as the forward power 116, the reverse power.118, the PA supply current 110, and the device temperature 114). During normal operation, all of the feedback signals except the forward power signal are relatively small. In this case, the controller 108 increases or decreases the power module setpoint signal 124 to regulate the forward power 116 of the power module 102. Under mismatched load conditions, another feedback signal, for example the supply current 110 to the power amplifier in the power module 102, dominates the forward power feedback 116. This will cause the controller 108 to reduce the power module setpoint 124. The power module 102 reduces the forward power delivered to the load 106.
This control technique is effective in protecting the generator from adverse loads but does not give repeatable performance when the cable length L between the power module 102 (the RF sensor 104) and the load 106 is varied. The change in the cable length L introduces a phase shift that may cause a high impedance load to be transformed into a low impedance load. The changes in the load impedance cause an increase or decrease in the current draw of the power amplifier in the power module 102. The impedance shift causes the supply current limiting loop of the power amplifier to reduce or increase the power module setpoint 124. This in turn causes the RF power generator control system 100 to deliver less or more power than the unphase-shifted case even though the voltage standing wave ratio (VSWR) has not changed.
In applications where repeatability is very important, such as semiconductor manufacturing, it may be very desirable to have a RF generator that has reduced sensitivity to cable length or load phase. For example, plasma delivery systems require precisely controlled conditions and repeatability. Some installations may have a longer distance from the chamber to the generator rack than others. Therefore, these systems will operate differently.
A radio frequency (RF) power generator system according to the invention includes a power generator that generates a RF power signal that is output to a load. The RF generator generates a forward power feedback signal and a reverse power feedback signal. A controller receives the forward power feedback signal and the reverse power feedback signal. The controller generates a setpoint signal that is output to the power generator. A setpoint modifier receives the forward feedback signal, the reverse feedback signal and an external setpoint signal. The setpoint modifier calculates a forward power limit based on the forward and reverse power feedback signals. The setpoint modifier and outputs a modified setpoint signal to the controller based on one of the forward power limit and the external setpoint signal.
In other features of the invention, the controller selects a lesser value between the forward power limit and the external setpoint signal. The power generator includes a RF sensor that generates the forward power feedback signal and the reverse power feedback signal. The power generator includes a power module that generates a supply current feedback signal and a temperature feedback signal that are output to the controller.
In still other features of the invention, the setpoint modifier and the controller are integrated. The setpoint modifier includes one of a lookup table and a formula for calculating the forward power limit. The formula determines the forward power limit based on the forward and reverse power feedback signals and maximum power dissipation.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention.